This invention relates to wet and dry strengthening resins which have the property of being substantially pH-independent and the paper treated with which can be easily repulped. More particularly, it relates to paper strengthening resins mainly composed of thermosetting polyvinylamide polymers which have the property of providing paper with a nearly constant wet and dry strength in the pH range of 3 to 9 in the case of methods of wet-end additions or in the pH range of 3 to 10 in the case of methods of surface applications, and moreover, the paper treated with which can be easily and rapidly repulped.
Paper is generally manufactured by beating pulp and mixing it with beater additives such as size agents, fillers and alum cake. Depending on the nature and amount of such additives, the pH of pulp slurry varies in different ways. In the case of using waste paper alone or using it together with virgin pulp as papermaking materials, a considerable amount of alkaline chemicals remain in the pulp slurry even after washing, because the waste paper is passed through a deinking process wherein alkaline chemicals are added. Consequently, the pH of the pulp slurry depends largely on the extent of washing and the proportion of waste paper used. In these cases, in order to use efficiently various other resins, it is necessary to adjust the pH value of the pulp slurry suitably for each prior art resin.
Various resins such as urea resin, melamine resin and polyamide-epichlorohydrin resin which impart wet and dry strength to paper are known in the art. However, both urea and melamine resins are efficient only in acid pH ranges but have low efficiency at neutral or alkaline pH. On the contrary, polyamide-epichlorohydrin resin imparts wet strength to paper in the alkaline pH ranges, but little wet strength in acid, and furthermore, not enough dry strength is obtained even in alkaline. The wet strength of paper treated with the above prior art resins is nearly permanent and therefore it is difficult to beat the paper even after long immersion in water and beating recovery is not easy.
Paper strengthening resins prepared by reaction of ionic or nonionic polyacrylamide polymers with glyoxal are disclosed in Japanese Patent Publications No. 24,926/1965 and No. 26,670/1969. In the Publication No. 26,670/1969, some methods of producing wet strengthening resins are disclosed, i.e., 2-dimethylaminoethyl methacrylate and acrylamide are polymerized and then the tertiary amino-group in the polymer is quaternized with methylene chloride, dimethyl sulfate or benzyl chloride to cationic polyacrylamide polymer and then the polymer is allowed to react with glyoxal. The paper strengthing resin thus obtained is efficient in the pH range of acid to neutral and as the pH value rises, the efficiency is gradually lowered. At above pH 8, the efficiency is significantly lowered and the resin consequently becomes of no practical use.
A wet strength resin mainly composed of the homopolymer of the cationic monomer prepared by reaction of 2-dimethylaminoethyl methacrylate with epichlorohydrin or copolymers of the cationic monomer with other vinyl monomers is disclosed in U.S. Pat. No. 3,678,098 and Japanese Patent Publication No. 29,126 of 1973. However, in order that the above homopolymer and copolymers may impart sufficient wet strength to paper, it is necessary that the copolymerization ratio of the cationic monomer be over 50 wt.% and this is not preferable from an economical point of view. Moreover, imparting sufficient wet strength to paper is not accomplished without subjecting the resin to caustic activation process just before use.